Various hearing aids have been described which are individualized for a specific person by casting its body in the auditory canal of this person or in a model of this person's ear canal. If this casting step is substantially the last process step and can be carried out simply, i.e. without complicated devices, this kind of hearing aid can be supplied to auditory advisors in a general condition, i.e. in a condition in which it has not yet been adapted to an individual auditory canal. The auditory advisor can adapt the appliance to the auditory canal of a specific person, i.e. cast the body, and deliver after only one session the finished appliance ready to be worn by the person.
Such simple individualization implies in particular that all components serving the hearing function (microphone and/or receiving coil, amplifier-chip and loudspeaker) are already integrated in the general hearing aid and that this general hearing aid comprises a casting cavity in which the individual body is cast such that the body does not require any further processing after casting.
An adaptation to an individual auditory canal is important for hearing aids worn partly in the auditory canal ("In-The-Canal-appliances" or ITC-appliances) and especially for hearing aids carried completely in the auditory canal ("Completely-In-the-Canal-appliances" or CIC-appliances). These appliances substantially have the form of an irregular frustum or cylinder with an inner face orientated towards the inside of the ear, an outer face orientated towards the outside and a circumferential surface, whereby the circumferential surface only has to be adapted to the form of the individual auditory canal.
Hearing aids which fulfil the requirements of a simple individualizability are e.g. described in publication EP-629101 and in a parallel application to the present application (Swiss application No. 1859/96). The hearing aids described in both cases comprise in their general condition a face plate with a battery opening defining the outer face and at least partly supporting elements which keep the loudspeaker in particular in a defined position relative to the front plate, i.e. at least within a defined distance from the face plate, such that these supporting elements and/or the outlet side of the loudspeaker substantially constitute the inner face of the appliance. The appliances further comprise a substantially tubular extensible membrane forming the circumferential surface which membrane is fastened around the outer face (face plate) and around the inner face (loudspeaker and/or further elements) such that they form a tight casting cavity together with the face plate and the loudspeaker and/or further elements in the region of the inner face, which cavity can be filled with a casting material through a pouring opening in the region of the face plate. The general appliance is positioned in the auditory canal of a potential wearer or in a corresponding model for casting. Through casting, the extensible membrane is pressed closely to the wall of the auditory canal such that the cast body of the appliance individualized in such a manner is adapted precisely to the auditory canal and carries the membrane on its circumferential surface.
A membrane forming the circumferential surface of a hearing aid must fulfill the most varied conditions:
The membrane must be extensible and flexible to such a degree that on casting it applies itself very closely to the wall of the auditory canal (or the wall of a corresponding model) without folds.
The membrane, as contacting element between hearing aid and skin, must be suited for constant and close contact with skin, i.e. it must be compatible with skin to a high degree.
The membrane must have a sufficient mechanical strength for the general appliance (with empty casting cavity, i.e. without body) to be able to be produced without problems and to be handled safely.
The membrane must be fastenable simply to the components of the hearing aid.
In addition, membranes which are so porous that they are permeable to air yet impermeable to a casting material are advantageous, not only for the casting of the body but also concerning the comfort when the hearing aid is worn.
For fixing the membrane to the face plate and to the loudspeaker and/or supporting element in the region of the inner face, clamping elements (fixing rings or locking elements with corresponding openings which elements reach over the whole faces) have been suggested or direct gluing or welding of the membrane to the face plate and to the supporting elements in the region of the inner face. It now appears that especially for CIC-appliances which are extremely small the fastening of the described membranes with the described fastening means and according to the described fastening methods is a step which is rather difficult to be carried out and is therefore costly. The reason for this is the sensitivity of the very thin (ca. 0.2 mm) membrane and the small size of the separate fastening means.